Nonaqueous electrolyte secondary batteries represented by lithium ion secondary batteries have a high energy density and a high capacity and are widely used as driving power sources of portable information terminals such as cellphones and notebook computers. Recently, nonaqueous electrolyte secondary batteries are increasingly demanded to be used in high-voltage and high-capacity applications such as driving power sources of battery-driven automobiles and household electricity storage systems. This trend has led to a need of such characteristics as high-output characteristics, long-term durability and safety.
Because of the use of combustible organic solvents, nonaqueous electrolyte secondary batteries are usually manufactured as sealed batteries. Sealed batteries have a risk, of rupture by the rapid generation of gas in the event of abnormalities such as internal short circuits, external short circuits, abnormal heating and abnormal impacts. As disclosed in, for example, Patent Literature 1, a known approach to solving this problem is to provide a safety valve in the seal of a cylindrical sealed battery and thereby to discharge a gas in the battery to the outside of the battery when the pressure inside the battery has reached a prescribed value.